USA Biology Olympiad (USABO) Practice Exam

During fertilization in animals, which process commonly occurs?

1. The fusion of a sperm nucleus with an egg nucleus.

2. The dispersal of gametes into the environment.

3. The formation of a zygote through binary fission.

4. The formation of a placenta before implantation.

The correct answer is: The fusion of a sperm nucleus with an egg nucleus.

The process of fertilization in animals fundamentally involves the fusion of gametes, specifically the sperm and egg cells. When a sperm successfully penetrates the egg, their nuclei merge, resulting in the formation of a diploid zygote. This single cell contains genetic material from both parents, initiating the developmental process that leads to the creation of a new organism. This fusion is critical because it restores the diploid number of chromosomes that are halved in the gametes through meiosis. It also sets the stage for subsequent cellular divisions and differentiation that transform the zygote into a multi-cellular organism. The other options describe processes or concepts that do not accurately reflect the fertilization event in animals. The dispersal of gametes into the environment refers to external fertilization mechanisms and does not represent the actual fusion event during fertilization. The notion of forming a zygote through binary fission pertains to a form of asexual reproduction, which is not relevant in the context of animal fertilization. Furthermore, the formation of a placenta before implantation is a process that occurs post-fertilization, specifically during the early stages of embryo development, making it unrelated to the fertilization process itself.